Because using assumptions as though they are proven facts is slander or libel. If you have any evidence that justifies what you say, it should go to the right authorities, not used as an online smear.
The Green party accused Genel Energy, of which Nadhim Zahawi, of Kurdish origin, is a major shareholder, of "laundering" Is oil.
The Green Party immediately retracted Clarke's comments. "The Green Party and Tony Clarke apologise to Genel Energy for this false statement, which they have withdrawn and have undertaken not to repeat," the statement said.
"The Green Party and Tony Clarke wish to publicly retract this statement and accept it to be untrue and without foundation. Genel Energy... is not, and never has been, a purchaser of oil from anyone."
Tony Clarke had Nadhim Zahawitold BBC Radio 5 on 17th Nov: "There are British oil companies such as Genel Energy, run by Nathaniel Rothschild, one of George Osborne's friends, who are making money, who are buying oil from ISIS, who are putting money into the pot, allowing ISIS therefore to fuel their evil across the world."
The Green Partry have undertaken not to repeat it. That means ALL members of the Green Party - and everyone else!